Gas sensors are useful for performing a variety of functions, and find application in many industries, including semiconductor fabrication, automotive, pharmaceutical, chemical, health care and environmental technologies. In all these industries, the common need is for an inexpensive, sensitive, selective and reliable gas sensor for detecting (and quantifying) one or more gaseous contaminants. The following discussion focuses on detection issues for the semiconductor fabrication industry, but the solid-state detector disclosed has application in all of the noted industries.
As feature size and overlay tolerances of semiconductor products continue to shrink, the impact of gaseous and particulate contamination increases. Herein the focal point is contamination within a wafer enclosure used, for example, to transport and/or hold wafers between processes. Of primary concern is film outgassing. In the case of resist-coated wafers, low vapor pressure gases are released over time at room temperature. These gaseous species can be adsorbed or absorbed on the wafers, they can diffuse through other films on a wafer, they can be adsorbed on enclosure walls, or they can leave behind precipitates anywhere within the enclosure including on the wafers. Over time, these residuals accumulate and the process can reverse such that gaseous and particulated materials can travel, for example, from an enclosure's walls to wafers within the enclosure.
Monitoring such contamination is complex. Considerations for each application include reliability, sensitivity, selectivity, installation and operation convenience, and cost. Ideally, every wafer enclosure will someday contain a detector continuously monitoring to detect extremely small quantities of materials of interest without interference from other species and yet be easy in operation to use and require minimal or no maintenance. The parametric field for such a detector is extensive; with possible contributions from gas residuals, precipitates from the gas phase, and particulation to the overall level of contamination.
One known approach to monitoring contamination levels is to employ an analytical detection technique, such as mass spectrometry, IR, and/or microbalance measuring. Such techniques, which are well described in the art, are generally expensive, cumbersome and are not in-situ or intended to function as a continuous monitor. Another approach to monitoring contamination levels is to employ a solid-state detector.
Various solid-state gas detectors are described in the open literature. For example, many prior art devices are based on semiconductor films, such as tin oxide (e.g., see U.S. Pat. Nos. 4,706,493 or 4,169,369). These films typically involve surface adsorption, diffusion of the gas through the bulk of the film, and finally an electrical response. Further, it has been proposed to mount a plurality of semiconductor thin films on a common substrate in a two-dimensional grid such that all films are in contact with the unknown gas and different films in the grid detect interference due to different gaseous species. Most solid-state detectors, however, have one or more disadvantages associated therewith, including low sensitivity (i.e., inability to detect gas of interest at desired concentration level), low selectivities (i.e., fail to detect a gas of interest in the presence of other gases), long-term drift, hysteresis effects, limited range of detectable gases, limited range of operating temperature and/or slow response time.
Accordingly, there exists a need in many industries for an enhanced solid-state gas detector (suitable, e.g., for monitoring contamination within an enclosure) having improved reliability, sensitivity, selectivity, installation and operation convenience, and a lower cost. The present invention provides such a solid-state gas detector.